...
I agree with you that Relic should provide durations and also reasons given for bans.
As I've been a mod here for quite a while now, I can tell you from experience that the clear cut examples are few and far in between. In the game it might be team killing, on forums or game chat personal attacks or threats of violence. But those are (luckily) not the majority of issues.
There's qualitative differences of people saying the same things. Context matters a lot. What's the topic of the thread, the current discussion, the posts that are being responded to. Even previous posting behaviour. An impolite post can potentially be different if posted by someone who spent his last 100 posts insulting others than from someone with a "clean sheet". Doesn't have to be, but the chances of malicious intent vary; yet, I have also have to give both the benefit of the doubt.
Then there's external factors: Non-native English speakers using wrong words, cultural differences of both poster and moderator themselves. Subconsciously also personal opinion, even if you try to avoid that to the best of your ability. To some degree, a moderator has to take all of thit into consideration, but still assume that everyone applies some basic common sense regardless of their background.
Lastly, as you've already stated in one of the first posts: No one can know all of this stuff. Either because it's literally impossible, or pragmatic reasons like time constraints. What if there is a probably insulting post, but I only have a minute to decide? I can leave it, hoping someone else with more time can have a look. If this doesn't happen, what then? I have to make a call, and they're wrong sometimes. I can make a judgement to the best of my abilities, but I'll still misjudge from time to time. Even the best guide lines will leave a lot of space for interpretation, and mindlessly applying words from guide lines will not make good moderation, either.
This is also what happens at Relic: They see a small excerpt of a chat, no info on the players, just a few lines of text. Maybe a message that this player has been reported another time this week. While I think that they should still write "You have been banned due to insulting other players" on their message in the main menu, I guess those won't solve much, because there is still a lot of grey area involved. How often have I seen posts in CoH and other games of users complaining how they got banned, with some later on literally stating stuff among the lines of "yeah I flame others if I think they play bad but that can't be a reason". Some might be troll posts, but there's legit idiots out there. Obviously everyone thinks they're in the 10% of misjudgements or due to mod bias or whatever. Might be, but chances are: they're not. They violated some rules, but are either too dumb or unwilling to realize and take the blame. It's much easier if someone else must be wrong. Still, false decisions can happen.
This makes it even more important, especially if there's bans of products you bought, that there are appeal mechanisms. Relic not offering any of this and basically saying "don't bother writing us" is poor. They can effectively lock you out of the money you spent on their game. It should be seen as a disadvantage to their product, but people don't see it as such until they get banned themselves.